A great number of customers are connected to an electric power system. In such an electric power system, it is desirable that the electric power to be consumed by the customers be stably supplied from the supplier. In order to realize the stable supply of electric power, there has been proposed a technique that prevents system down of the electric power system by preparing for power outage, instantaneous power interruption, etc. that might occur in the near future by employing a duplexed (redundant) power feeding system, as described in JP, A 2007-172535, for example.